1. Field of the Invention
The present invention relates to optical frequency routing devices for multiplexing individual wavelength channels of a multiple laser wavelength system and, in particular, to methods and apparatuses for multiple laser wavelength channel stabilization in such a system.
2. Description of the Related Art
Optical transmission systems employing fiber optic cables are often used to transmit data by means of optical signals. Wavelength division multiplexing (WDM) is sometimes used in such systems to increase the capacity of such fiber optic transmission systems. In a WDM system, plural optical signal channels are carried over a single silica based optical fiber with each channel being assigned a particular wavelength. Dense WDM (DWDM) is also increasingly being used.
Erbium-doped optical fiber amplifiers (EDFAs) are often used to amplify light in optical transmission systems. For example, EDFAs are used to transmit amplified optical signals to the input of an optical fiber, or to amplify light received from an optical fiber. EDFAs contain a single-mode optical fiber doped with erbium. The erbium-doped fiber is xe2x80x9cpumpedxe2x80x9d with light at a selected wavelength to provide amplification or gain at wavelengths within a low loss window of the optical fiber. The input light pump for an EDFA is typically provided by a pump laser module comprising a semiconductor laser diode (pump laser) plus an associated lens system.
A pump laser module typically comprises a pump laser such as a semiconductor laser diode fabricated in a given substrate such as InP or GaAs; an optical lens system for focusing and optically processing the beam; and a fiber for receiving the beam and outputting the beam. The pump laser typically receives an input signal in the form of an electrical current, and outputs an optical beam on the fiber. This fiber is typically fusion-spliced into a single-mode fiber of a wavelength division multiplexing (WDM) device. The fiber of the WDM device may be fusion-spliced to a single-mode erbium doped optical fiber of an optical amplifier. The WDM device combines the pump light and signal light and outputs this to the single-mode erbium doped optical fiber of the optical amplifier. The optical amplifier thus receives an optical signal at a relatively low power level, and amplifies this signal to provide an output optical signal at a higher power level.
Frequency routing devices, in particular, are often used to perform optical switching, multiplexing, and demultiplexing. These functions have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with an optical grating comprising a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbor by a predetermined fixed amount. The grating is connected to the input of a second star coupler, the outputs of which form the outputs of the switching, multiplexing, and demultiplexing apparatus. An example of such an interconnection apparatus is disclosed in U.S. Pat. Nos. 5,002,350 and 5,136,671.
The geometry of such an apparatus may be such that a plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, a plurality of input wavelengths is directed to a predetermined one of the input ports of the apparatus. Each of the input wavelengths is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelength also permits switching between any selected input port to any selected output port. Accordingly, these devices are referred to as frequency routing devices.
U.S. Pat. No. 5,412,744, xe2x80x9cFrequency Routing Device Having a Wide and Substantially Flat Passband,xe2x80x9d issued May 2, 1995 (Dragone), the entirety of which is incorporated herein by reference, describes a technique for producing a flat passband in a frequency routing device used as a wavelength multiplexer.
Optical techniques and components, such as EDFAs and planar lightguide circuit (PLC) DWDMs make it possible to increase the data-carrying capacity of fiber optic transport systems. There is typically a fixed transmission rate per wavelength. Thus, to increase the capacity as much as possible, it is desired to increase the number of channels (wavelengths) used. This, in turn, leads to the need to decrease the channel-to-channel wavelength (or frequency) spacing. However, the decrease in frequency spacing means that the light sources (laser transmitters) employed need to be more carefully controlled so as not to drift from one wavelength channel to another.
Typically, it is desirable to have lasers with wavelength drifts much less than the channel width; larger drifts would result in increasing modulation distortion as the modulated signal approaches the edge of the system filters (which may have also drifted), and in increased interchannel leakage in the DWDM demultiplexing functions. Minimizing such channel drift may be referred to as wavelength stabilization. Fiber optics, waveguide technology, DWDM, and related matters are discussed in Yuan P. Li and Charles H. Henry, xe2x80x9cSilicon Optical Bench Waveguide Technology,xe2x80x9d in Ivan P. Kaminow and Thomas L. Koch, eds., Optical Fiber Telecommunications IIIB (Academic Press, 1997): ch. 8 (pp. 319-376); Yuan P. Li and Leonard G. Cohen, xe2x80x9cPlanar Waveguide DWDMs for Telecommunications: Design Tradeoffs,xe2x80x9d NFOEC ""97 Proceedings, pp. 365-374 (1997), the entireties of each of which are incorporated herein by reference.
U.S. Pat. No. 5,745,275, xe2x80x9cMulti-Channel Stabilization of a Multi-Channel Transmitter Through Correlative Feedback,xe2x80x9d issued Apr. 28, 1998 (Giles et al.), the entirety of which is incorporated herein by reference, is directed to a method for monitoring and stabilizing the power in each optical channel. However, the Giles et al. patent does not teach a way to monitor and stabilize the wavelength, As noted above, wavelength stabilization is increasingly important for higher channel density applications. Each laser source may be separately stabilized. This approach, however, would require the use of a potentially costly or impractically large number of passive optical components. There is, therefore, a need for wavelength stabilization in optical systems employing multiple wavelengths.
An optical apparatus for multiplexing a plurality of optical signals of different wavelengths to provide a multiplexed output optical signal. A plurality of input waveguides receive the plurality of optical signals. A first free space region is connected to the plurality of input waveguides, and an optical grating comprising a plurality of unequal length waveguides is connected to the first free space region. A second free space region is connected to the optical grating, and an output waveguide is connected to the second free space region for providing the multiplexed output optical signal. First and second cross-coupling output ports are connected to the second free space region for providing first and second cross-coupling output signals representative of the cross-coupling of the optical signals with said cross-coupling output signals, wherein said first and second cross-coupling signals are approximately equally strong for a given one of the optical signals only when said optical signal has a wavelength substantially equal to a specified wavelength for said optical signal.